Kelvin Duncan, a biologist at the University of Canterbury, responded.

Plants grow so that they can become big enough to make seeds so
that the species can continue even when the parent plant dies.

Unlike animals, plants make their own food. They capture the energy
in sunlight and use it to make organic food from minerals, water and
carbon dioxide (one of our waste products). This is called photosynthesis
and is one of the great mysteries of nature. We know a lot about it,
but by no means everything.

Plants use their food for growing bigger, for making other plants of
the same kind, and for carrying out the very important functions that
keep them alive. In this process the food they made by photosynthesis
is broken down to water, carbon dioxide and minerals.

Once the plant has grown to a certain size it can slow down its growth
and start making seeds. This ensures the survival of the species.
The seeds cannot each be as big as the parent, so seeds are small but
they grow once germinated.

Following on from your question is the amazing observation that most
plants seem to make far more food than they need. For some strange reason
most plants make up to ten times more food than they require. This allows
animals, like us, who cannot make their own food, to have an abundant
supply without taking all the plants food in the process. Plants make so
much that there seems to be plenty for all, both plant and animals.